What you are after
The value of a good should be the highest possible utility that it would give anyone in the economy. If you care differently about different people, you are interested in the highest possible adjusted utility that it would give anyone in your economy, where you have adjust the utility with a weight that indicates how much you comparatively care about that person
It doesn't exist
This is almost what a price is: It is the highest possible utility that it would give anyone in the economy who can afford it, under frictions. These frictions could be informational (someone who benefits most from the new iPhone isn't aware of its existence) or other (some people/groups are restricted from purchasing a good, for example due to trade sanctions).
There is no other objective way that we are aware of. At the cost of sounding dogmatic, this is the superior part of capitalism compared to other economic systems: Allowing prices to judge the value of goods.
In alternative systems, you need committees or similar to do this job. While there is no theoretical reason why human's couldn't do a better job than prices do (unless you allow for hand waving the system is too complex), it turns out they don't.
Do you need real world cash for prices?
There is three properties of money: It is
- a storage of value
- a unit of exchange
- unit of account
Introduction of money into your economy doesn't need to connect it to real world cash, but you need to make it valuable. Connecting it to real world cash at some rate would immediately determine its value. Say, if you allow modwizdollar at 1-to-1 rate to the USD, your dollar has immediately the same value as the dollar, and also its inflation rate.
In Diablo, players find gold and items over time, as they kill enemies. Therefore, the objective value of gold is the time it takes a player to earn that much gold, devalued by how much that time is worth to the player. The more time it takes for players to find items, the more valuable these are. For money to be work as a store of value, it is sufficient to keep finding modwizdollars costly in terms of time.
How to set prices
It depends on what your goals are, and I suppose people could write Bachelor papers on the complexity of these systems, and in fact, an economist of late publicity did exactly that for Valve.
One simple way to set up prices
One simple way would to allow the measure of goods (items) grow constantly over time. This would imply a new supply of a specific item $i$ of $S_i$. Let time be denoted by $t$.
Compute, at any time, the actual demand for goods by players given the current prices $D_i(p_{i,t})$. Given initial $p_{i,t}$,
- Increase $p_{i,t+i}$ if $D_i(p_{i,t}) > S_i$
- Decrease $p_{i,t+i}$ if $D_i(p_{i,t}) < S_i$
How much you want to increase or decrease these depends on many things, and I can't possibly spell out the whole model for you. Assuming that demands are stable over time, you could try a bijection.
